Metal-halide arc discharge lamps are well known. They are frequently employed in commericial usage because of their high luminous efficacy and long life. See IES Lighting Handbook, 1981 Reference Volume, Section 8.
The terms "efficacy" or "luminous efficacy" used herein are a measure of the total luminous flux emitted by a light source over all wavelengths expressed in lumens divided by the total power input of the source expressed in watts. The terms "maintenance" or "luminous maintenance" herein denote the ratio of the illuminance on a given area after a period of time to the illuminance on the same area by the same lamp at an initial or benchmark time; the maintenance ratio is a dimensionless number usually expressed as a percentage.
A typical commercial metal halide arc discharge lamp comprises a quartz or fused silica arc tube hermetically sealed within a borosilicate glass outer envelope. The arc tube, itself hermetically sealed, has tungsten electrodes sealed into its ends and contains a fill comprising mercury, metal halide additives, and a rare gas to facilitate starting. The outer envelope is generally filled with nitrogen or another inert gas at less than atmospheric pressure.
One problem associated with metal halide lamps is sodium loss from within the arc tube. Most metal halide lamps contain a sodium compound as one ingredient of the arc tube fill. It has been postulated that during operation of the lamp, a photoelectric process caused by a flux of ultraviolet radiation emitted from the arc tube and incident upon the frame parts liberates electrons which migrate to and collect on the arc tube. The electrons on the outside of the arc tube create an electric field which draws sodium ions through the arc tube walls into the atmosphere of the outer envelope. This process depletes the sodium from within the arc tube causing diminished efficacy and maintenance and, ultimately, reduced lamp life. For a more detailed explanation of sodium loss, see Electric Discharge Lamps, by John F. Waymouth, The M. I. T. Press, 1971, Chapter 10, and further references cited therein.
Another problem, which is associated with metal halide lamps having a phosphor coating on the inside of the outer envelope, is the reaction of the phosphors with reducing agents. The phosphors used in high intensity discharge lamps are limited to very stable phosphors, such as the orthovanadates, because of the high ambient temperatures. The orthovandates, being metal oxides, are subject to being reduced by the presence of a reducing agent, such as hydrogen, in the atmosphere of the outer envelope. This causes an accelerated loss of phosphor efficiency and increases phosphor absorption of emitted light due to darkening.
Yet another problem experienced with metal halide lamps is the possibility of striking an electrical arc between the lead-in wires of the external circuit. This "arc-over" problem is especially significant when the atmosphere of the outer envelope is at low pressure, e.g., between 50 microns and 10 torr. For a more detailed explanation of the arc-over problem, including typical Paschen curves showing ignition potential as a function of fill pressure for various gases, see Light Sources, by W. Elenbaas, Crane, Russak & Co., Inc., New York, 1972.
Still another problem of metal halide lamps is heat loss from the arc tube by means of convective currents within the atmosphere of the outer envelope. It is generally true that the overall efficiency of a metal halide lamp is improved with higher operating temperatures of the arc tube wall. Higher operating temperatures cause greater quantities of the metal halide additives to be in the vapor state. An excess of the additives is usually provided to insure a saturated vapor state within the arc tube. With more vaporized additives, the luminous output and color temperature of the lamp are improved in most cases. Therefore, it is important to keep heat lost through convection at a minimum.
In metal halide lamps of lower wattage, e.g., 100 watts or less, avoidance of convective heat loss is a principal concern. Consequently, lamp manufacturers have been constrained to have a vacuum or near vacuum in the outer envelope despite the possible benefits which would be concomitant with greater fill pressures.
In metal halide lamps of higher wattage, e.g., 175 watts or higher, convective heat loss is not so critical as to compel a near vacuum in the outer envelope. These lamps generally contain an outer envelope fill having cold pressure of approximately one-half of an atmosphere. Nevertheless, convective heat loss adversely affects the efficacy and luminous maintenance of these lamps.
In U.S. Pat. No. 4,281,274, issued July 28, 1981, to Bechard et al., there is disclosed a glass shield surrounding the arc tube of a metal halide arc discharge lamp. It is suggested that the shield prevents sodium loss from the arc by trapping ultraviolet radiation and by shielding the arc tube from photoelectrons.